This invention relates to a transmission power control method and, more particularly, to a transmission power control method in a wireless communication system for controlling transmission power on the transmitting side in such a manner that measured reception quality will agree with a target reception quality.
In W-CDMA (Wideband-Code Division Multiple Access)mobile communications, multiple channels are distinguished from one another by spreading codes assigned to the channels, thereby allowing communication by multiple channels sharing a single frequency band. In an actual mobile communications environment, however, a receive signal is susceptible to interference from its own channel and from other channels owing to delayed waves ascribable to multipath fading and radio waves from other cells, and this interference has an adverse influence upon channel separation. Further, the amount of interference sustained by a receive signal varies with time owing to momentary fluctuations in reception power ascribable to multipath fading and changes in the number of users communicating simultaneously. In an environment in which a receive signal is susceptible to noise that varies with time in this fashion, it is difficult for the quality of a receive signal in a mobile station linked to a base station to be maintained at a desired quality in a stable manner.
In order to follow up a change in number of interfering users and a momentary fluctuation caused by multipath fading, inner-loop transmission power control is carried out. In such control, the signal-to-interference ratio (SIR) is measured on the receiving side and the measured SIR is compared with a target SIR, whereby control is exercised in such a manner that the SIR on the receiving side will approach the target SIR.
Inner-loop Transmission Power Control
FIG. 9 is a diagram useful in describing inner-loop transmission power control. Here only one channel of the system is illustrated. A spread-spectrum modulator 1a of a base station 1 spread-spectrum modulates transmit data using a spreading code conforming to a specified channel. The spread-spectrum modulated signal is subjected to processing such as orthogonal modulation and frequency conversion and the resultant signal is input to a power amplifier 1b, which amplifies this signal and transmits the amplified signal toward a mobile station 2 from an antenna. A despreading unit 2a in the receiver of the mobile station applies despread processing to the receive signal and a demodulator 2b demodulates the receive data. A SIR measurement unit 2c measures the power ratio between the receive signal and an interference signal and a comparator 2d compares target SIR and measured SIR. If the measured SIR is greater than the target SIR, the comparator 2d creates a command that lowers the transmission power by a TPC (Transmission Power Control) bit. If the measured SIR is less than the target SIR, on the other hand, the comparator 2d creates a command that raises the transmission power by the TPC bits. The target SIR is a SIR value necessary to obtain, e.g., 10−3 (error occurrence at a rate of once every 1000 times). This value is input to the comparator 2d from a target-SIR setting unit 2e. A spread-spectrum modulator 2f spread-spectrum modulates the transmit data and TPC bits. After spread-spectrum modulation, the mobile station 2 subjects the signal to processing such as a DA conversion, orthogonal modulation, frequency conversion and power amplification and transmits the resultant signal toward the base station 1 from an antenna. A despreading unit 1c on the side of the base station applies despread processing to the signal received from the mobile station 2, and a demodulator 1d demodulates the receive data and TPC bits and controls the transmission power of the base station 1 in accordance with a command specified by the TPC bits.
FIG. 10 is a diagram showing the structure of an uplink DPCH (Dedicated Physical Channel) frame standardized by the 3rd Generation Partnership Project (referred to as “3GPP” below). There is a DPDCH channel (Dedicated Physical Data Channel) on which only transmit data is transmitted, and a DPCCH channel (Dedicated Physical Control Channel) on which control data such as a pilot and TPC bit information, described above with reference to FIG. 9, is multiplexed. After each of these is spread by an orthogonal code, they are mapped onto real and imaginary axes and multiplexed. One frame of the uplink has a duration of 10 ms and is composed of 15 slots (slot #0 to slot #14). The DPDCH channel is mapped to an orthogonal I channel and the DPCCH channel is mapped to an orthogonal Q channel. Each slot of the DPDCH channel consists of n bits, and the n varies in accordance with the symbol rate. Each slot of the DPCCH channel that transmits the control data consists of ten bits, has a symbol rate of 15 ksps and transmits a pilot PILOT, transmission power control data TPC, a transport format combination indicator TFCI and feedback information FBI.
Outer-loop Transmission Power Control
Owing to changes in traveling velocity during communication and changes in the propagation environment ascribable to travel, the SIR that is necessary to obtain a desired quality (the block error rate, or BLER) is not constant. It should be noted that BLER is the ratio of the total number of transport blocks (TrBk) over a fixed period of time to the number of transport blocks TrBk in which CRC error has occurred over this period.
In order to deal with these changes, the BLER is observed and control is exercised so as to increase the target SIR if the observed value of BLER is inferior to the target BLER and decrease the target SIR if the observed value of BLER is superior to the target BLER. Control that thus changes the target SIR adaptively in order to achieve the desired quality is well known as outer-loop transmission power control (outer-loop TPC).
FIG. 11 is a block diagram of well-known outer-loop control. According to this scheme, a signal that has been transmitted from the base station 1 is decoded by an error correcting decoder 4a after it is demodulated by the demodulator 2b. The decoded signal is then applied to a CRC detector 4b where it is divided into transport blocks TrBk and subsequently subjected to CRC error detection on a per-TrBk basis. The result of error detection applied to each transport block TrBk is sent to target-SIR controller 4c. 
In W-CDMA as currently standardized, encoding is performed on the transmitting side in the manner shown in FIG. 12. Specifically, if a plurality (N) of transport blocks TrBk exist in a unit transmission time (Transmission Time Interval, or TTI), a CRC add-on circuit on the transmitting side generates a CRC (Cyclic Redundancy Code) error detection code for every transport block TrBk and adds this onto the transmit data. An encoder on the transmitting side joins the N-number of transport blocks TrBk having the attached CRCs and encodes the blocks by error correcting coding such as convolutional coding or turbo coding. On the receiving side the error correcting decoder 4a subjects the receive data to error-correction decoding processing and inputs the result of decoding to the CRC detector 4b, and the CRC detector 4b performs CRC error detection for every transport block TrBk constituting the result of decoding and inputs the results of error detection to the target-SIR controller 4c. 
Immediately after a dedicated channel DCH (Dedicated CH) call is placed to the target-SIR controller 4c, a host application specifies the required BLER that conforms to the service type of the DCH, such as voice, packet or unrestricted digital. In outer-loop control, let BLERquality represent the required BLER, let Tmax represent the number of transport blocks TrBk for which BLER is measured, let Sinc (dB) represent an update quantity for raising the target SIR in a case where the measured BLER is inferior to the required BLER, and let Sdec (dB) represent an update quantity for lowering the target SIR in a case where the measured BLER is superior to the required BLER. If there is even one CRC NG (CRC error) in Tmax-number of BLER measurement periods, the target SIR is updated by Sinc. If CRC OK holds throughout, the target SIR is updated by Sdec. When this is observed in total, the target SIR settles stabilizes at a fixed level. This is the fundamental concept of outer-loop control. According to this concept, the values Sinc, Sdec and Tmax are decided so as to satisfy the following equation:(1−BLERquality)Tmax×Sdec=[1−(1−BLERquality)Tmax]×Sinc   (1)It should be noted that (1−BLERquality)Tmax indicates the probability that the CRC check will be correct Tmax-times in succession, and [1−(1−BLERquality)Tmax] indicates the probability that there will be even one CRC check error in Tmax times.
More specifically, BLER measurement is performed with regard to Tmax-number of transport blocks TrBk. If CRC OK is obtained for all TrBk, the target SIR is updated by Sdec. If there is even one CRC NG (CRC error), then the target SIR is updated by Sinc. The values of Sinc, Sdec and Tmax are values uniquely decided by the required BLER of each service.
Relationship Between Service Quality and SIR Update Interval
Transforming Equation (1), we have the following:Tmax=log {1/[1+(Sdec/Sinc)]}/log(1−BLERquality)  (2)
The value of BLERquality satisfies the relation 0<BLERquality<1, the numerator on the right side is a negative value at all times, and the denominator on the right side also is a negative value at all times. Therefore, the higher the required quality of the BLER, i.e., the lower the value of BLERquality, the greater the value of Tmax. For example, if Sinc=0.1 dB, Sdec=−0.3 dB holds, then Tmax=270 will be obtained when BLERquality=5×10−2 holds and Tmax=277224 will be obtained when BLERquality=5×10−5 holds. Thus, the higher the quality of the required quality of BLER, the greater the value of Tmax.
Control of Target SIR
The initial value of the target SIR is the same for all bearers (all services). A point that is positively above a convergence point (the convergence target SIR) is set as the initial target SIR beforehand. At the moment a CRC result concerning the receive signal appears after the connection of a call, the target SIR is updated based upon this result. More specifically, if an error is not detected whenever a CRC check is performed, the target SIR is reduced a prescribed value at a time starting from the initial target SIR. When an error is detected, from this point onward the error rate is measured at a target-SIR update period T that conforms to the service, this measured error rate is compared with a required error rate and the target SIR is updated accordingly. For example, in case of a TrCH for which TTI Transmission Time Interval)=10 ms holds, a CRC result is ascertained every 10 ms and the target-SIR update period becomes 10 ms. The incremental amount of updating is made a relatively large value so as to detect CRC NG quickly, e.g., a value on the order of −1 dB, which is a value that is ten times the usual. Outer-loop control from the initial value of the target SIR to detection of CRC NG shall be referred to as the “initial state”.
After detection of CRC NG, the values of Sinc, Sdec, Tmax are calculated from the required BLER (=BLERquality), which is specified by the host application, using Equation (2), and the target SIR is updated at T (=Tmax×TrBk period). For example, if BLERquality=5×10−2 holds, Sinc=0.1 dB, Sdec=|−0.3|dB=0.3 dB, Tmax=270 hold and all CRC results up to the point where 270 transport blocks TrBK are counted are OK, then target SIR is updated by −0.3 dB. If a CRC result is NG (No Good) for even one transport block TrBk, then the target SIR is updated by 0.1 dB. This control for updating target SIR shall be referred to as the “steady state”.
Method of Calculating Measured SIR
In the spread receive-symbol data, the power of the DPCH (Dedicated Physical Channel) of the local station is the desired wave power and is defined as DPCH_RSCP [RSCP: Received Signal Code Power (dBm)]. Further, interference waves of a common pilot channel (CPICH: Common Pilot Channel) of other stations not orthogonal to the local DPCH and of the DPCH_RSCP of other stations are defined as ISCP [Interference Signal Code Power (dBm)]. Furthermore, total power (referred to as overall reception power) with respect to all receive signals obtained by despreading the common pilot channel (CPICH) of local/other stations and the dedicated physical channel (DPCH) is defined as RSSI (Received Signal Strength Indicator).
Measured SIR is calculated in accordance with the following equation:SIR=(DPCH—RSCP−ISCP)×SF (dB)  (3)where SF represents the spreading factor of the code and is a value of from 4 to 512. Since Equation (3) is a logarithmic expression, it can be written as follows:SIR=(DPCH—RSCP/ISCP)×SF (dB)  (3)′
The downlink (the link from the base station to the mobile station) DPCH frame has a frame period of 10 ms, which is divided into 15 slots, as shown in FIG. 14. Each slot is composed of 2560 chips, and the number of bits per slot is as indicated by the following equation:Tslot=10×2k bits (k=0, 1, 2, . . . , 7)  (4)
Further, the spreading factor SF and k are related as follows:SF=512/2k  (5)
If a signal is received at a high bit rate per unit time 10 ms (i.e., if k is large), the number of chips per bit will be small and the spreading factor SF will be small. Conversely, if a signal is received at a low bit rate, the number of chips per bit will be large and the spreading factor SF will be large.
The spreading factor SF in Equation (3) has a constant value from the connection of a call to the end of the call. The measured SIR is obtained by measuring the value of DPCH_RSCP and the value of ISCP and performing the calculation of Equation (3).
In a case where the measured SIR is found to be higher than the target SIR upon comparing the value of the measured SIR and the value of the target SIR, transmission power control information to the effect that the transmission power is to be lowered is inserted at a prescribed position (the TPC bits) of the DPCCH from the mobile station to the base station. Conversely, if the measured SIR is lower than the target SIR, then transmission power control information to the effect that the transmission power is to be raised is inserted. Transmission power control in the downlink direction is performed upon inserting the proper transmission power control information.
Other Prior Art
In transmission power control, it is necessary to measure the SIR of the receive signal correctly. To accomplish this, the specification of Japanese Patent Application Laid-Open No. 2003-32168 proposes weighting the interference power of each path, thereby measuring the interference power precisely to achieve highly precise SIR measurement.
Further, the specification of Japanese Patent Application Laid-Open No. 2003-18089 proposes varying the amount of updating of a target value adaptively in accordance with changes in the propagation environment, thereby maintaining a desired reception quality irrespectively of the magnitude of any change in propagation environment.
Further, the specification of Japanese Patent Application Laid-Open No. 2003-78484 proposes shortening the time it takes to achieve convergence following the start of transmission power when downlink transmission power control has been carried out.
Problem at Time of Decline in Power Allocated to Pilot Bits
The value of DPCH_RSCP used in the calculation of Equation (3) is a value of the power of the receive-signal DPCH pilot bits of the local station, and the value of ISCP is Equation (3) indicates the value of interference power of other stations, etc. When a propagation environment in which the interference of other stations is small and the value of ISCP is comparatively low is considered, the value of DPCH_RSCP becomes predominant in the measured value of SIR.
FIG. 15 is a table useful in describing slot formats of the downlink DPCH. The table illustrates, for every slot format specified by a slot format number, the relationship between spreading factor SF and number of data bits, TPC bits, TFCI bits and pilot bits per slot and the proportion occupied by the spreading factor SF. The smaller the spreading factor SF, the smaller the ratio of the pilot bits occupying one slot. For example, whereas the pilot ratio is 10 to 40% up to SF=32, the pilot ratio falls to 5% or less from SF=32 onward.
Downlink power (the transmission power of the base station) can be varied on a per-slot basis by TPC control on the side of the mobile station. This will be considered while excluding the effects thereof. Further, it will be assumed that the initial value of downlink power is constant even in a case where the spreading factor SF differs, and that there is no power offset of DPCCH (TPC, TFCI, Pilot) with respect to DPDCH power. In such case the downlink power per slot will be constant even in a case where the spreading factor SF differs and therefore the power allocated to the pilot bits will be proportional to the pilot ratio.
For example, assume that the power of one slot is Pslot. If SF=512 holds, the pilot ratio will be 40% and therefore pilot-bit power will be 0.4×Pslot. However, if SF=4 holds, the pilot ratio will be 1.25% and therefore pilot-bit power will be 0.0125×Pslot.
In this case, the pilot-bit power at SF=4 gives rise to a difference of 10 log10(0.0125×Pt)−10log 10(0.4×Pt)=−15 dB in comparison with SF=512.
In actuality, when the spreading factor SF becomes small, there is a tendency for the power allocated to the pilot bits to diminish, though there is a difference between a state in which data is packed in the DPDCH and a state (Discontinuous Transmission, or DTX) in which there are portions with no data. In a case where the power allocated to the pilot bit is small, the value of power in the pilot portion of the DPCCH develops a corresponding amount of error. This causes an error in the.measured value of SIR. By way of example, if it is assumed that the ISCP value is constant in Equation (3), the receive signal at SF=4 will incur an SIR measurement error (SIR variance) of 15 dB in comparison with the situation at SF=512 on the assumption that there is no DTX of the data portion.
Downlink power when such SIR variance occurs will be considered.
Let T represent the duration of measurement, BLERquality the required BLER over this period, Sinc_total the total update value on the + side in the measurement period T and Sdec_total the total update value on the − side in the measurement period T. Sinc, Sdec represent the amount of update per time, (1−BLERquality)T is the probability that CRC NG will not appear at all over the time T, and 1−(1−BLERquality)T is the probability that CRC NG will appear one or more times over the time T, and therefore Equation (6) below holds. (Sdec, which is the update quantity on the − side, represents the absolute value of the actual amount of decrease. For example, Sdec=|−0.1|=0.1 in case of −0.1 dB.)Sdec_total/Sinc_total={[(1−BLERquality)T]×Sdec}/{[(1−(1−BLERquality)T)]×Sinc}  (6)
In the measurement period T, control that causes the measured value of BLER to agree with the required BLER (=BLERquality) is performed and BLERquality takes on a value that satisfies the inequality 0<BLERquality<1. As a result, the content of the power of (1−BLERquality)T in Equation (6) takes on a value smaller than 1. The larger T, therefore, the smaller the value of (1−BLERquality)T.
Accordingly, when T is large, the value of Sdec_total/Sinc_total is small. Conversely, when T is small, Sdec_total/Sinc_total is large. In other words, if power allocated to pilot bits diminishes and SIR variance occurs, then the smaller T, the larger Sdec_total.
The fact that T is small means that the observation period of BLER is short, or in other words, that updating of target SIR is performed frequently in order to acquire the required BLER when there is a large SIR variance. In this case, the value of Sdec_total/Sinc_total becomes large. That is, the total value of decrease update on the − side is larger than the total value of increase update on the + side. This is equivalent to saying that because the downlink power results in excessive quality, the value of measured BLER takes on a quality higher than that of the required BLER and a greater amount of decrease updating is applied.
Thus, if variance (SIR variance) ascribable to SIR measurement error becomes excessive, a problem which arises is that downlink power (transmission power) becomes too large and excessive quality is the result. It is apparent from Equation (6), this tendency becomes more pronounced for bearers (services) for which the value of BLERquality is small, i.e., for high-quality bearers (services). This represents a first problem of the prior art.
Problem Relating to Outer-loop Power Control According to the Prior Art
In order for a user to communicate immediately after a call is connected, the necessary control data is set and received frequently between the mobile station and the base station. More specifically, control information to the effect that call connection is to be performed is transmitted from the side of the base station to the side of the mobile station on the DCCH (Dedicated Control Channel). The DCCH delivers the information to a plurality of TTIs (TTI=40 ms), though this depends upon the amount of information for the purpose of call connection.
However, in the initial state of outer-loop control immediately after call connection, control is exercised in such a manner that updating of the target SIR toward the − side is carried out rapidly, whereby the steady state is attained upon causing the appearance of CRC NG. At this time the DCCH control data undergoes Viterbi encoding and becomes a single shot of data, while the DTCH (Dedicated Traffic Channel) data undergoes turbo encoding and is continuous. DTCH data includes UDI (Unrestricted Digital signals) and packets, etc. Consequently, the target SIR attains the stable state upon exceeding a CRC-NG occurrence level LC of the DCCH, in which the data is a single shot, and arrives at a level LD at which CRC NG occurs on DTCH, where the data is continuous. Since the DCCD control data is sent and received when the initial state prevails, there is a possibility that some of the necessary data will not be acquired depending upon the communication bearer (service) in the region where the target SIR is less than LC. This represents a second problem encountered in the prior art.
Another Problem Relating to Outer-loop Power Control According to the Prior Art
Consider a case where communication is performed in an environment in which there is a sudden change, measured SIR declines on rare occasion in a shadowing state or the like and measured BLER deteriorates rapidly. In the shadowing state, in which radio waves are interrupted by a building BL, as shown in FIG. 16(A), the target SIR suddenly increases. Then, when the building is passed, the target SIR ideally decreases rapidly. In actuality, however, the target SIR is updated every update time T. In the case of a bearer (service) for which T is short, therefore, the target SIR diminishes in a comparatively short time. However, in the case of a high-quality service for which T is long, a long period of time is required for the target SIR to decline, as indicated by the shading in FIG. 16B. A problem which arises is that there is excessive demand for downlink power (transmission power) during this time. This is a third problem of the prior art.
Thus, the first to third problems cited above remain unsolved in the prior art.